How would that happen for parallel cells? Li-ion cells are charged via a current limited constant voltage regime.

So, if you had two cells that start off charged they would discharge over time as you use them. They would discharge and maintain the same cell voltage since they are in parallel. Maybe one cell has less charge capability than the other, but who cares, they will maintain the same cell voltage.

Now you go to charge them, as long as the charger can put in no more current than the spec for each cell, then current will be shared across the two cells and never exceed the maximum current for each cell. The charger will at most apply 4.20V (unless you have some junk charger) and it is safe to apply 4.20V to both cells for a long period of time.

How would that happen for parallel cells? Li-ion cells are charged via a current limited constant voltage regime.

So, if you had two cells that start off charged they would discharge over time as you use them. They would discharge and maintain the same cell voltage since they are in parallel. Maybe one cell has less charge capability than the other, but who cares, they will maintain the same cell voltage.

Now you go to charge them, as long as the charger can put in no more current than the spec for each cell, then current will be shared across the two cells and never exceed the maximum current for each cell. The charger will at most apply 4.20V (unless you have some junk charger) and it is safe to apply 4.20V to both cells for a long period of time.

Feel free to educate me.

cheers,
george.

George is right. The only consequence of mismatched cells, in terms of capacity, in a 1SxP pack is the current may not split evenly. So you won't get into trouble as long as the max discharge and charge current is never higher than what's allowable for a single cell. That's almost always at least 1C for a cell and sometimes is higher. Check the specs on the cells.

George is right. The only consequence to mismatched cells in a 1SxP pack is the current may not split evenly. So you won't get into trouble as long as the max discharge and charge current is never higher than what's allowable for a single cell. That's almost always at list 1C for a cell and sometimes is higher. Check the specs on the cells.

So if you start out with equally charged batteries with the same capacity, then all would be good, right?

So if you start out with equally charged batteries with the same capacity, then all would be good, right?

Actually, we may be mixing up terms here. There is "capacity" and there is "state of charge". Capacity is the mah rating of the cells. Such as 2600 mah. "State of charge" is the current percentage the cell is charged up.

When building a pack with series cells, such as a 4S1P pack, the cells must be equal both in term of capacity and state of charge for the pack to be well balanced. For a 1SxP pack that only has parallel cells, then you can get away with cells that are mismatched to some extent in both capacity and state of charge. Keep in mind that capacity changes with the number of cycles and age, so cell may have different capacities because of a different usage history if they aren't new unused cells made at a similar time.

I suspect folks in this thread have used the term capacity when they really meant state of charge. For a 1SxP pack, the initial state of charge only matters the instant you initially connect the cells together. Take the worse case scenario, if one cell was at 10% SOC, and another cell is at 100% SOC, when you connect them together in parallel the fully charged cell will instantly start charging the nearly discharged cell. The problem is there is nothing to limit the current assuming these are unprotected cells. If they are protected cells, this will cause the protection PCBs in one or both of the cells to trip. And they typically will not reset until the cells are disconnected or put back on the charger depending on the specifics of the protection PCB. If the cells are only mildly different in terms of SOC, then the higher cell will charge the lower cells and by definition the pack will become perfectly balanced.

If you are talking about a 1SxP pack with cells of different capacities, that also works, but then you really need to adhere to the current limit of a single cell as I said in the earlier post. That's because on discharge, the cells with the lowest capacities will stop contributing current when they are fully discharged. So at the end of the cycle, only the cell with the highest current will be contributing current and that current needs to be below that cell's limit. The same thing happens in reverse for the charge cycle.

If you're building a 1SxP pack with new cells, from the same manufacturer, specs, and preferably from the same production batch, then you need to make sure the initial SOC is somewhat close just to limit any high current when you connect the cells together. After that, the pack will perfectly self balance. Also, since the cells should be fairly closely matched in terms of capacity, internal resistance, and other factors, the current should split fairly evenly between the cells. In that case, it's ok for the charge and discharge currents to be above the specs for a single cell. It would be a good idea to limit the current to the max specs of half the cells in the pack. Then you would be safe even if there were some current imbalance resulting from difference in the cells as they age.

If you're building a 1SxP pack with new cells, from the same manufacturer, specs, and preferably from the same production batch, then you need to make sure the initial SOC is somewhat close just to limit any high current when you connect the cells together. After that, the pack will perfectly self balance.

That´s what I will do.
First, fully balance charge the 3 or 4 cells as a 3S1P/4S1P pack on my balance charger, and THEN connect them 1S4P.

While were on the subject of battery packs and chargers..
Is this a good charger for packs?

http://www.giantcod.co.uk/power-bala...-p-402624.html
I normally charge my 18650's in two of the DX 2 cell chargers. At the moment i have a 7.4v 5000mah and 14.8v 2500mah, using turboferrets 18650 holders.
Im assuming that chagrer could charge them if they were packs with decent matched cells etc.
could i charge a 14.8v 5000mah? 4S1P?

14.8V 5000mAh in 4SP1 is not possible with 18650 cells, simply because cells with 5000 mAh capacity are not available. Probably you ment 4S2P?

Originally Posted by Goldigger

edit 50w limit.. so it wont charge a 14.8v 5000mah

There is no reason why 50W charger could not charge 14.8V 5000 mAh pack. 4SxP pack, when fully charged, has a voltage of around 17V. To get 50W of power with this voltage, you need almost 3A of current. Now, you probably won't charge your pack with such a high current. Set the charger to charge with, say, 2A or even lower charging current and it will be more than happy to charge your pack....

14.8V 5000mAh in 4SP1 is not possible with 18650 cells, simply because cells with 5000 mAh capacity are not available. Probably you ment 4S2P?

Yes...

There is no reason why 50W charger could not charge 14.8V 5000 mAh pack. 4SxP pack, when fully charged, has a voltage of around 17V. To get 50W of power with this voltage, you need almost 3A of current. Now, you probably won't charge your pack with such a high current. Set the charger to charge with, say, 2A or even lower charging current and it will be more than happy to charge your pack....

Ok i rewatched the video and it said that the max you can charge at is 5A so bigger packs wont be able to be charged at 5A...

I have iMax B6 (actually a cheap Chinese clone of it) and it's OK. I'm not doing any RC, I'm only using it to charge my light packs, and for that it's OK.

Not at the moment, it just seemed a better option as it didnt have the 50w limit.. and was cheaper than the first one i suggested.. I just want a decent charger for my light packs, i dont have any RC either

I'm going to start another thread...
as i would like to know how to build my packs, connectors needed etc..rather than hijack this thread.

Not at the moment, it just seemed a better option as it didnt have the 50w limit.. and was cheaper than the first one i suggested.. I just want a decent charger for my light packs, i dont have any RC either

I'm going to start another thread...
as i would like to know how to build my packs, connectors needed etc..rather than hijack this thread.

Cheers

the 50W limit is just the max charge current it can supply for a given voltage - so you'll be able to charge a 2S pack at 5A easy (5 x 7.4 = 37.1) but you'll only be able to charge a 4S pack at 50/14.8 = 3.3A. Given that those charge currents are way higher than most mtbers charge their battery packs at, it's not an issue.

finally found link again, JST-XH connectors are easy to find of eBay, Hobbyking and even Farnell (you'll probably have to wire the plugs up yourself though).